1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to trip units providing zone interlocking for such apparatus. The disclosed concept further pertains to electrical switching apparatus providing zone interlocking. The disclosed concept also pertains to systems providing zone interlocking.
2. Background Information
Circuit interrupters, such as for example and without limitation, circuit breakers, are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a short circuit, or another fault condition, such as an arc fault or a ground fault. Molded case circuit breakers typically include a pair of separable contacts per phase. The separable contacts may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to a detected fault condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip mechanism, such as a trip unit, which senses a number of fault conditions to trip the breaker automatically. Upon sensing a fault condition, the trip unit trips the operating mechanism to a trip state, which moves the separable contacts to their open position.
Typically, two or more circuit interrupters are placed between a fault and the source of the fault current. In order to minimize electrical service interruption, the circuit interrupters are selective in response such that the one nearest the fault will first attempt to interrupt the fault current. If this circuit interrupter does not timely clear the fault, then the next upstream circuit interrupter will attempt to do so, and so on. This response selectivity is sometimes termed system selective coordination.
Zone selective interlocking (ZSI) (e.g., also known as “zone interlocking”) is a method of controlling circuit interrupters in order to provide selectivity with relatively very short delay times, irrespective of the number of zones (e.g., without limitation, a line side zone; a load side zone; a number of upstream zones; a number of downstream zones; a number of grading levels) and the location of a fault in a power distribution system. A ZSI input (ZSI_IN) and a ZSI output (ZSI_OUT) are provided at each circuit interrupter. Interlocking may be applied to faults between phases or earth-faults or both.
As one example, zone interlocking uses a ZSI_OUT/ZSI_IN communication scheme to connect line and load circuit interrupter trip units together. When a fault occurs, the trip units communicate using a ZSI_OUT/ZSI_IN hardwired connection to determine which load side circuit interrupter is closest to the fault. The trip unit in the circuit interrupter closest to the fault overrides any customer-defined delay and opens instantaneously, thereby clearing the fault and allowing the line side circuit interrupters to remain closed.
If ZSI is used in several zones, then each circuit interrupter affected by, for example, a short circuit current (i.e., upstream of the fault) interrogates the circuit interrupter(s) directly downstream of that affected circuit interrupter to determine whether the short circuit current is present in or is affecting the adjacent downstream zone. A delay setting tZSI is adjusted at each circuit interrupter to ensure that the downstream circuit interrupter, directly upstream of the fault, has time to interrupt the fault current. The advantages of ZSI increase with additional zones, since time-based selectivity can result in unacceptably long delays at the upstream power source end of the system.
Each of the circuit interrupters in a distribution system is typically sized in order that they can achieve selective coordination. More often than not, upstream circuit interrupters have to be oversized.
Although the use of ZSI_IN and ZSI_OUT signals help to accomplish certain aspects of coordination, they require hardwiring between downstream and upstream circuit interrupters.
There is a need for short circuit zone location detection that allows circuit interrupters to act autonomously.
There is room for improvement in trip units for electrical switching apparatus.
There is also room for improvement in electrical switching apparatus.
There is further room for improvement in systems including electrical switching apparatus.